Facsimile signals representing graphic original material are generated by scanning the graphic original line by line to produce a rectangular array of generally parallel horizontal lines known as a raster familiar on television screens. Each line of the raster is a series of analog or digital pulses. The facsimile recorder which receives the facsimile signals prints them line by line on sensitive paper fed by a linear printing head.
Teletype (Trademark) signals represent alphanumeric characters of a predetermined configuration as compared to the random graphic elements of a facsimile weather map, for example, and are transmitted in a seven bit on-off code with a combination of five to twelve pulses for each character. Such coded on-off pulses would be recorded unintelligibly if applied to a conventional facsimile recorder because they are not in raster form.
Further recording problems arise not only because of differences in coding but also because of different modes of modulating the coded signals for transmission over radio and land lines.
Teletype signals are conventionally transmitted in a first form of frequency modulation termed frequency shift keying, wherein a signal is modulated between two frequencies or tones which are separated by a known amount and are relative to a carrier frequency, wherein the ON or MARK portion of a code pulse is represented by the frequency or tone closer to the carrier frequency, whereas the OFF or SPACE portion of a code pulse is represented by the second frequency or tone further from the carrier frequency. Teletype signals are usually printed by a teletype machine specifically designed to accept the Teletype code and frequencies.
Facsimile signals, as compared to teletype signals, may represent various tone shades or density levels of the original graphic material in the range from white to black with intermediate gray levels. The signals may therefore be transmitted as a relatively wide band of frequencies representing the various white-to-black shades of gray. However, some facsimile recorders, thermal printers for example, are capable only of marking thermal recording paper with near-black marks separated by unmarked white spaces.
Graphic signals are thus transmitted in these different pairs of frequency bands for Teletype and another different frequency band for facsimile presenting a problem of how to receive and process both Teletype and facsimile signals and print them on a facsimile recorder, and it is the object of the present invention to receive both forms of signals and print them on a facsimile recorder, particularly a thermal recorder for marking paper which contrasts only black and white tones.